The term "cook-off" as used herein means the exposure of a propellant to an external source of heat that can lead to unplanned ignition, explosion, or chemical deterioration of the propellant to such an extent as to self destruct, made dangerous to handle, or rendered ineffective for its intended function. Fast "cook-offs" may occur when a propellant charge is exposed to a flame environment such as may be encountered by missiles carried by an aircraft that is hit by enemy fire and resulting in a fire in the aircraft fuel tanks. Slow "cook-offs" may occur when a missile is stored in an area of increased temperature such as in a bunker and influenced by heat from a fire in an adjacent bunker.
Most current tactical composite propellants contain ammonium perchlorate (AP) as the main oxidizer. Ammonium perchlorate containing propellants often respond quite violently to unplanned-for hazards, especially in cook-off scenarios. Hence, it would seem desirable to replace ammonium perchlorate with a more thermally stable oxidizer. However, in a slow cook-off scenario a higher oxidizer decomposition temperature may not always imply a reaction of lessened violence. Instead, what might be anticipated is a longer interval until thermally-induced damage, self-heating and violent "whole mass" participation of the propellant is realized. Hence, a small proportion of a less thermally stable ingredient might be incorporated into the formulation to function as a "thermal trigger". Thus, early, mild initiation of the propellant and preemption of a severe slow cook-off reaction may be accomplished.
Potassium perchlorate (KP) is highly preferred among the existing thermally-stable oxidizers due to its ready availability, relatively low cost and ease in processing. For potassium perchlorate propellants, ammonium perchlorate has been demonstrated to be an excellent "trigger". At 15-20% ammonium perchlorate levels, these propellants exhibit good processability and mechanical properties while offering reduced sensitivity to cook-off. They also show a mild performance gain as potential candidate propellants for certain surface-to-surface and surface-to-air missiles. The disadvantage, however, lies in an increase in motor weight since potassium perchlorate, like most other thermally-stable oxidizers, is significantly more dense than ammonium perchlorate. It is also desirable, and in many cases necessary, to decrease the relatively high burning rate exponent often exhibited by potassium perchlorate propellants.
As the ammonium perchlorate content of potassium perchlorate based propellants increases (through a trade-off with potassium perchlorate), propellant specific impulse (performance) increases, while its density (motor weight) decreases. The propellant burning rate exponent also decreases with increasing ammonium perchlorate content. On the other hand, a higher ammonium perchlorate content yields a higher propellant cook-off response as the ammonium moves away from its role of "thermal trigger" into one of dominant oxidizer.
Accordingly, it is an object of the present invention to provide a potassium perchlorate propellant composition having an increased quantity of ammonium perchlorate beyond the level normally tolerated by such a propellant before a significant loss of cook-off advantage occurs.
Another object of the present invention is to add a small quantity of iron oxide to a potassium perchlorate/ammonium perchlorate propellant formulation to reduce the relative violence of the propellant reaction during cook-off.